There is a polyphase inverter having a control circuit (referred to as a “high side control circuit”) for driving a high arm switching device and a control circuit (referred to as a “low side control circuit”) for driving a low arm switching device separately. In addition, a so-called bootstrap method in which a power source for driving the high side control circuit is obtained from a capacitor, which is charged through the low arm switching device has been proposed. Refer to, for example, the following patent documents 1, 2 and non-patent document 1.
Patent Document 1: Japanese Patent Application Laid-Open Gazette No. 2003-348880
Patent Document 2: Japanese Patent Application Laid-Open Gazette No. 2004-304527
Non-Patent Document 1: Mitsubishi Electric Corporation, “Mitsubishi HVIC Application Note” [online], [searched on Jun. 30, 2005], Internet <URL: http://www.mitsubishichips.com/Japan/files/manuals/km0020a1.pdf>
In the bootstrap method, a period (referred to as a “precharge period”) for charging the above-described capacitor for bootstrap (referred to as a “bootstrap capacitor”) is set before performing a normal operation, and in this period, while the high arm switching devices are turned off, the low arm switching devices are sequentially turned on and a charge (referred to as a “precharge”) of the bootstrap capacitor is performed.
FIG. 6 is a timing chart showing the precharge period in a three-phase inverter and on/off of the switching devices in a motor driving time following the same. For example, the low arm switching devices in U-phase, V-phase and W-phase are turned on in this order for every 500 μs in the precharge period. When a cycle in which the low arm switching devices in the three-phase are turned on is set to one cycle, the precharge period is 30 cycles, for example, that is to say, over a length of 45 ms.
When the precharge period is finished, the high arm switching devices of U-phase, V-phase and W-phase and the low arm switching devices of U-phase, V-phase and W-phase are turned on/off by a pulse width modulation control, for example, for a normal motor driving.